This invention relates generally to apparatus and methods for sensing physical phenomena and particularly to fiber optic sensing systems. This invention relates particularly to fiber optic sensors that respond to changes in a selected field quantity such as pressure, magnetic field, electric field, etc. Still more particularly, this invention relates to fiber optic interferometric sensors that respond to underwater perturbations such as acoustic wavefronts by producing a phase difference in two light beams propagated by fiber optic material.
Optical fibers can be made sensitive to a large number of physical phenomena, such as acoustic waves and temperature fluctuations. An optical fiber exposed to such phenomena changes the amplitude, phase or polarization of light guided by the fiber. Optical fibers have been considered for use as sensing elements in devices such as microphones, hydrophones, magnetometers, accelerometers and electric current sensors.
A hydrophone array or acoustic sensor array may be formed as an integral, self-contained linear array of hydrophones on a single cable. Conventionally, such an array is made up of electromechanical transducer elements, principally piezoelectric devices, which generate electrical signals in response to pressure variations. These conventional sensors typically are active devices that require many electrical wires or cables. These sensors have the disadvantage of being susceptible to electrical noise and signal cross talk.
Fiber optic Mach-Zehnder and Michelson and interferometers respond to the phenomenon being sensed by producing phase differences in interfering light waves guided by optical fibers. Detecting phase changes in the waves permits quantitative measurements to be made on the physical quantity being monitored.
A fiber optic Mach-Zehnder interferometer typically has a reference arm comprising a first length of optical fiber and a sensing arm comprising a second length of optical fiber. The sensing arm is exposed to the physical parameter to be measured, such as an acoustic wavefront, while the reference arm is isolated from changes in the parameter. When the Mach-Zehnder interferometer is used as an acoustic sensor, acoustic wavefronts change the optical length of the sensing arm as a function of the acoustic wave pressure amplitude. An optical coupler divides a light signal between the two arms. The signals are recombined after they have propagated through the reference and sensing arms, and the phase difference of the signals is monitored. Since the signals in the reference and sensing arms had a definite phase relation when they were introduced into the arms, changes in the phase difference are indicative of changes in the physical parameter to which the sensing arm was exposed.
A Michelson interferometer also has a sensing arm and a reference arm that propagate sensing and reference signals, respectively. However, in the Michelson interferometer these arms terminate in mirrors that cause the sensing and reference signals to traverse their respective optical paths twice before being combined to produce an interference pattern.
A hydrophone array is typically towed behind a ship. Towing causes vortexes, bubbles and other disturbances in the water that cause conventional hydrophones to give erroneous outputs. Most fiber optic hydrophones employ a matched Mach-Zehnder interferometer in the acoustic sensing system. One arm of the interferometer senses the acoustic field while the other arm is a reference. With a matched interferometer the reference arm can be placed next to the sensing arm so that any mechanical stresses applied to the sensing arm will also be applied to the reference arm.
A coating is applied to the jacket of the optical fiber in the reference arm to keep it from being sensitive to the acoustic field being measured. However, attempting to make the optical fiber in one arm of the interferometer insensitive to a particular physical parameter may also change other properties of the optical fiber. For example, coating the fiber jacket of one arm of the interferometer to change its sensitivity to the acoustic field changes the sensitivity of the optical fiber to acceleration. In such cases the effect of having matched arm lengths is not an advantage. Another difficulty with the matched pathlength interferometer is that coating on the fiber jackets do not completely eliminate the sensitivity of the fiber to the acoustic field, but leave a residual sensitivity that affets the performance of the sensor.